The present disclosure relates to a technology of a developing roller that is disposed to face, without contact, the outer circumferential surface of the image carrier.
A developing device is installed in an image forming apparatus which is a copier, a printer or the like and forms an image on a paper sheet based on the electrophotography. The developing device develops, by toner, an electrostatic latent image formed on an image carrier such as a photoconductor drum. As the developing method, a so-called two-component developing method is known which uses two-component developer including magnetic carrier and toner to develop a toner image on the image carrier. As an example of the two-component developing method, there is conventionally known a contactless developing system called “interactive touchdown developing system”. In the interactive touchdown developing system, a developing roller and a magnetic roller are used. The developing roller is disposed at a predetermined distance from the image carrier. A magnet is embedded in the magnetic roller. The magnetic roller draws up the magnetic carrier as well as the toner, and holds them on the surface thereof. The magnetic roller forms a magnetic brush thereon to transfer only the toner to the developing roller, and form a toner thin layer on the developing roller. An AC electric field is generated by a developing bias that includes an AC component applied to the developing roller, and the AC electric field flies the toner from the developing roller and causes the toner to adhere to the electrostatic latent image on the image carrier.
There is known a developing roller used in this type of developing device, wherein in the developing roller, a base body made of aluminum is coated with an alumite layer, and the alumite layer is coated with a resin coat layer. The alumite layer plays a role in suppressing a leak from occurring between the image carrier and the developing roller.
The alumite layer of the developing roller is an aluminum oxide coating film formed on the surface of the base body by an electrochemical treatment (called “alumite treatment” or “anodic oxidation processing”) where the base body of aluminum, as a positive electrode, is dipped into an electrolytic tank containing acidic aqueous solution, and a current is supplied thereto. During the process of forming the resin coat layer on the surface of the alumite layer, the developing roller is subjected to a high-temperature environment (for example, from 90° C. to 130° C.) to dry the resin coat layer. Alumite is easy to generate cracks when it is laid in a high-temperature environment, due to a difference in thermal expansion coefficient from aluminum, which is a raw material of alumite. When cracks occur during the drying process of the resin coat layer, resin enters the cracks. This causes the resistance value of the developing roller to change, and the insulation performance of the developing roller changes. The cracks are different in number and size for each developing roller. Thus, the amount of resin that has entered the cracks of the alumite layer is different in each developing roller, and as a result, the insulation performance varies for each developing roller. The cracks are present deep in the alumite layer and the tips of the cracks function as electrodes. This becomes a cause to break the insulation of the alumite layer and generate a leak. On the other hand, there has been conventionally known a configuration for suppressing variation in performance of the developing roller, wherein the resin coat layer is directly formed on the base body of the developing roller, without forming the alumite layer on the base body.